On Tuesday, the 18th Of October 2016 at 14h00, Fabien RORTAIS from CEA (DRF/INAC/SPINTEC), will defence his thesis “Study of injection and detection in silicon and germanium: From local measure of accumulation to non-local detection of pure spin current”



Place : Grenoble INP Ense³, site GreEn-ER
21 Rue des Martyrs, 38000 Grenoble



Since the discovery of the giant magnetoresistance in 1988 by the group of Albert Fert (Nobel Prize in 2007), the field of spintronics has been growing very fast due to its potential applications in micro-electronics. For almost 20 years, it has been proposed to introduce the spin degree of freedom directly in the semiconducting materials. Spintronics aims at using not only the charge of carriers (electrons and holes) but also their intrinsic spin degree of freedom.
In that case, spins might be manipulated with electric fields. By using both charge and spin, one might add new functionalities to traditional micro-electronic devices. Indeed, the first challenge of semiconductor spintronics is to create and detect a spin polarized carrier population in traditional semiconductors like Si and Ge to further manipulate them. For this purpose, we have used hybrid ferromagnetic metal/insulator/semiconductor devices which allow us to perform electrical spin injection and detection.
The first part of this thesis deals with 3 terminal devices grown on different substrates and in which a single ferromagnetic electrode is used to inject and detect spin polarized electrons using the Hanle effect. A spin signal amplification is measured experimentally as compared to the value from the theoretical diffusive model, this raised a controversy concerning 3 terminal measurements. We demonstrate that localized defects in the tunnel barrier cannot be at the origin of the measured MR signal and spin signal amplification. Instead, we show that the presence of interface states is the origin of the spin signal amplification in all the substrates. By using a proper surface preparation and the MBE growth of the magnetic tunnel junctions, we reduce the density of interface states and show a significant modification of the spin signals.
In a second part, we present the transition from 3 terminal measurements to lateral spin valves on semiconductors.
In the last configuration by using two ferromagnetic electrodes, charge and spin currents are decoupled in order to avoid any spurious magnetoresistance artefacts. Using epitaxially grown magnetic tunnel junctions we can prove the spin injection in silicon and germanium. Especially, we are able to measure non local spin signals in germanium up to room temperature. Finally, we study the spin Hall effect in gallium arsenide and germanium substrates. For this propose we induce spin accumulation using the spin Hall effect (i.e spin-orbit coupling) and probe it using muon spectroscopy. We demonstrate, at low temperature the presence of spin accumulation by the coupling between nuclear spins and the electron spin accumulation.



Jury :
– Pr. Michel Hehn, Institut Jean Lamour, Nancy (rapporteur)
– Pr. Pierre Renucci, INSA Toulouse (rapporteur)
– Dr. Federico Bottegoni, Politecnico di Milano, Milan (examinateur)
– Dr. Joel Cibert, NEEL, Grenoble (examinateur)
– Dr. Pierre Dalmas de Réotier, INAC, Grenoble (examinateur)
– Dr. Jean-Marie George, UMPhy CNRS-Thales, Palaiseau (examinateur)
– Dr. Matthieu Jamet (directeur)